1. Field of the Invention
The present invention relates to a film forming method in which a photosensitive ink film is formed by screen printing onto a plate material during the manufacture of printed wiring boards.
2. Description of the Related Arts
FIG. 3 illustrates a screen printing step in the process of manufacturing printed wiring boards. A screen 33 is tensed against a substrate 30 which includes an insulating plate 31 laminated with a copper foil 32, and a squeegee 34 slides across the screen 33. A photosensitive ink 35 which is supplied in front of the squeegee 34 drops through the screen 33 as the squeegee 34 slides across the screen 33 in the direction of the arrow, thereby forming a photosensitive ink film 36 over the entire surface of the substrate 30. After the formation of the photosensitive ink film 36, an exposure is carried out through a photomask, the photosensitive ink corresponding to an unexposed region is removed by development, and then the copper foil in the region free from the photosensitive ink is removed by etching to form an electrically conductive pattern acting as a circuit.
In the production of printed wiring boards as described above, a broad plate material is usually used as a substrate. The plate material is subjected to screen printing over the entire surface thereof, and subsequently to the exposure and etching to obtain a multiplicity of printed wiring boards at one time. Thereafter the plate material is cut into printed wiring boards. FIG. 4 shows a broad plate material 40. Screen printing is executed over the entire surface of the plate material 40 to form a photosensitive ink film thereon. In order to cut the plate material 40 into printed wiring boards in a subsequent step, a plurality of regions known as cutting spaces or cutting areas 41 (represented by broken lines) are longitudinally and transversely provided.
When the entire surface of the plate material 40 is subjected to screen printing, however, there arises a problem that the photosensitive ink film is liable to be uneven in thickness. In other words, the photosensitive ink film forms thinner in the region tear the center of the plate material as shown by dashed line hatching 42 in FIG. 4. The thinner film portion thus formed prevents the exposure, development, and etching in later steps from being uniformly performed, which is an obstacle in the manufacture of printed wiring boards.
The present invention was conceived in view of the above disadvantages, and an object is to provide a method of film formation in screen printing wherein a photosensitive ink film having an even thickness can be provided over the entire surface of a broad plate material.